Photodissociation and rotational excitation of interstellar CO.

Details Photodissociation and rotational excitation of interstellar CO. Photodissociation and rotational excitation of interstellar CO.

Apr 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...308..535w&link_type=abstract

Astronomy and Astrophysics, v.308, p.535-564

Astronomy and Astrophysics

Astrophysics

67

Ism: Molecules, Ism: Clouds General, Molecular Processes, Radio Lines: Ism

Scientific paper

We have developed a detailed interstellar cloud model that takes into account the chemistry among simple carbon and oxygen bearing molecules, as well as ^13^C and ^18^O isotopic compounds. The rotational population of H_2_, CO, ^13^CO and C^18^O and the fine structure excitation of C, C^+^ and O are controlled by chemical processes, including selective photodissociation which depends on the rotational quantum number J, collisional and radiative processes allowing for photon trapping of CO millimeter and sub-millimeter lines. The gas and grain temperature distributions are obtained by solving a thermal equilibrium equation for each component. Comparison with the previous model calculations of van Dishoeck & Black (1988) and Le Bourlot et al. (1993) are made throughout the paper. We have computed series of models for three kinds of quiescent interstellar clouds: diffuse, translucent and dense dark clouds. The variations of the photodissociation rates are mainly controlled by self-shielding effects leading to a rotational selective photodissociation, i.e. a strong dependence on the rotational quantum number J: the more populated the level the more efficient is the self-shielding and the more rapid is the decrease of its photodissociation rate. This implies a strong coupling between photodissociation and rotational excitation, which has for main consequence to overpopulate low-lying levels and to under populate excited levels with respect to an LTE calculation. Under the combined influence of the rotational photodissociation and the inefficiency of collisions to populate excited levels, at the densities and temperatures prevailing in most clouds, the rotational population of CO and its isotopes is sub-thermally excited, except for the first two rotational levels of the three isotopes and the levels J=2 and 3 of the main isotope which are thermalized in the inner part of dark clouds. Photo-electric emission of electrons from grains is the dominant heating process in regions where UV photons are present, i.e. in diffuse and translucent clouds and in the outer parts of dark clouds. It is replaced by cosmic ray ionization of atomic and molecular hydrogen as a dominant heating process in the core of dark clouds. The cooling efficiency is entirely correlated to the cloud composition, with cooling dominated by fine structure excitation of C^+^ and rotational excitation of CO in diffuse and dark clouds, respectively; fine structure excitation of C contributes to the cooling of translucent clouds. The strong dependence of the temperature distribution in interstellar clouds on the external UV radiation field is emphasized. Our model calculations have been confronted with selected CO surveys of molecular clouds by Frerking et al. (1982), Cernicharo & Guelin (1987) and Lada et al. (1994). For that purpose, we have run series of model with various visual extinctions and densities covering a large range of values, from diffuse to dense cloud conditions. The temperature distribution was obtained by solving the thermal balance equation. The model predicts emissivities of the 1->0 line of CO well below the observations with the exception of the C^18^O(1->0) line in the the molecular clouds surveyed by Frerking et al. (1982) for which agreement between theory and observations is very good. Although the model predicts too low abundances of both ^13^CO and C^18^O, the column density ratio R(^13^CO/C^18^O) normalized to the isotopic ratio that we compute is in fairly good qualitative agreement with the one derived by Lada et al (1994) in the IC 5146 molecular cloud. The model calculations have also been used to predict the emissivities of the CO, ^13^CO, C^18^O, C and C^+^ lines under various interstellar conditions.

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